The present invention relates to a semiconductor electro-optical monolithic component comprising at least two sections each having a wave guide etched in the form of a strip and buried in a cladding layer.
For this type of buried strip multi-section electro-optical component, it is important to have a high electrical isolation between each section in order to avoid interactions between these during the operation of the component. The invention relates more particularly to any electro-optical component, comprising at least one transmitting element and one receiving element which are integrated, for which it is sought to allow a simultaneous transmission/reception operation, without any interaction between the transmitter and the receiver.
FIG. 1 depicts a diagram in longitudinal cross-section of a conventional in-line transmitter/receiver component, denoted IL TRD ("In-line Transmitter Receiver Device"), obtained by monolithic integration of a laser 30 and a detector 20 on one and the same substrate 10. The laser 30 transmits a signal towards an optical fibre 50 for example, while the detector 20 receives a signal coming from this same optical fibre. The transmitting wavelength of the laser 30 is less than the receiving wavelength of the detector 20. For example, the transmitting wavelength is equal to 1.3 .mu.m while the receiving wavelength is equal to 1.55 .mu.m. In this case, given that the transmitting wavelength is less than the receiving wavelength, and that the laser 30 is situated close to the detector 20, the laser can cause optical interference on the detector. This is because the laser also transmits, in the direction of the detector, light at 1.3 .mu.m which dazzles the said detector. In order to avoid this dazzling of the detector, the component has a third section, disposed between the laser 30 and the detector 20, forming an optical isolator 40. This optical isolator makes it possible to absorb the light transmitted at 1.3 .mu.m in the direction of the detector, so that the latter can detect the optical signal at 1.55 .mu.m coming from the optical fibre without being interfered with by the laser.
The substrate 10, or lower layer, can for example be of n-doped InP. The wave guides respectively 21 of the detector 20 and 31 of the laser 30 and of the optical isolator 40 are etched in the form of strips and buried in a strongly doped cladding layer 11. The wave guides are of so-called BRS ("Buried Ridge Structure") type. The cladding material 11 is p.sup.+ -doped when the substrate is n-doped. Of course, this type of strip is only an example. Other types of strip can be suitable. The n and p dopings of the different layers can also be reversed.
There are many variants of composition and dimensions of the wave guides. In the example of FIG. 1, the wave guide 21 of the detector 20 is for example implemented in ternary material, while the wave guides 31 of the laser and of the optical isolator 40 are implemented with one and the same quantum-well structure.
Moreover, metal electrodes 22, 32, 42 and 13 are formed on the different sections and on the underneath of the component, so as to allow it to operate.
On account of the presence of conductive layers (11), the component also has electrical isolation areas I, or resistive areas, between the different sections 20, 30, 40 in order to avoid any electrical interference of one section with regard to another during operation of the component.
This type of in-line transmitter/receiver, having a central part 40 allowing absorption of all the light flux transmitted at 1.3 .mu.m towards the detector, works very well for all light which is guided in the wave guide strips 31.
However, not all the light transmitted is completely guided. This is because there is also spontaneous light which is transmitted in the whole volume of the component. In addition, part of the stimulated light can also be diffracted in the component as a result of the presence of defects in the wave guide 31.
The curves in FIG. 2 reveal the penalties noted on the detector sensitivity, in dB, for different operating indices. Curve A depicts a receiving reference when the laser is off, curve B depicts a receiving reference when the laser is on continuously and curve C depicts the simultaneous modulation of the laser and the detector. A 4.5 dB penalty between curve B and curve C, when the laser and the detector are modulated simultaneously, is noted. This penalty is mainly optical. It is caused by the non-guided light transmitted at 1.3 .mu.m, in all directions, which interferes with the detector at 1.55 .mu.m. This stray light attacks the detector mainly through the lower part (the substrate) of the component, that is to say through the n-doped lower layer 10 situated under the guiding layer strips 21, 31.
This optical interference coming from the substrate 10 is depicted very schematically in FIG. 1. A metal electrode 13, disposed at the substrate/air interface, can act as an optical reflector. Part of the spontaneous light transmitted in the volume of the component can therefore be reflected by the electrode 13 and return to couple with the detector 20 from underneath. This is why the stray light, which is coupled via the substrate of the component, has been depicted, in FIG. 1, by a wave 60 reflected on the metal electrode 13 of the substrate. Of course, the interference of the detector 20 by the non-guided light is in reality much more complex than a single reflection. This is because part of the stray light can also undergo multiple reflections in the lower layer 10. Another part of this stray light can also dazzle the detector at a grazing incidence for example.
Some techniques have already been envisaged to combat the 4.5 dB penalty noted in the example given in FIG. 2, which occurs at the time of simultaneous modulation of the laser and the detector. The techniques envisaged are electronic techniques. They consist for example in taking part of the laser modulation signal, and then in subtracting it at reception. The use of these electronic processing techniques has demonstrated a 2 dB reduction in the penalty. However, they require the devising, manufacture and development of special electronics for this particular type of transmitter/receiver component, with the result that they considerably increase the cost of this component. However, it is sought to manufacture this type of component on a large scale and therefore reduce its production cost as much as possible. Consequently, these electronic processing techniques cannot be used for the mass production of such a component.
Moreover, an in-line transmitter/receiver is intended to be installed at the premises of subscribers and it must be able to operate between around 0 and 70.degree. C. without any temperature regulation. However, the reliability of these electronic techniques has not been demonstrated over this temperature range and it is not proved that they can automatically adjust as a function of the temperature.